1. Field of the Invention
This invention relates to a process for the production of lower alkyl oligoglucosides in which glucose sirup is added at elevated temperature to a mixture of a short-chain alcohol and an acidic catalyst via an inline mixer, the water of reaction is azeotropically distilled off and the reaction mixture is subjected to an after-reaction.
2. Statement of Related Art
Surface-active long-chain alkyl oligoglucosides have long been known as raw materials for the production of detergents. They are normally produced by acid-catalyzed acetalization of glucose with long-chain fatty alcohols. In the processes known as the "transacetalization process" or, in special cases, as the "butanol route", the glucose is reacted with a short-chain alcohol, for example butanol, in a first step to form the corresponding glucoside which, in a second step, is subjected to transacetalization with a fatty alcohol. Accordingly, lower alkyl oligoglucosides, such as butyl glucosides for example, are important intermediate products for the production of long-chain alkyl oligoglucosides. In addition, they are directly used as solubilizers and emulsifiers.
European patent application EP 0 319 616 A1 describes a process for the production of lower alkyl oligoglycosides in which an aqueous sugar solution is mixed with a short-chain alcohol, an acidic catalyst is added to the homogeneous solution and the mixture is subsequently reacted at 60.degree. to 200.degree. C. and preferably at 80.degree. to 150.degree. C., any water present being azeotropically distilled off.
International patent application WO 90/1489 describes the reaction of glucose sirups having a DP 1 content (i.e. monomer content) of less than 90% by weight with butanol. The acetalization is carried out with intensive stirring in an aqueous system at temperatures above 125.degree. C. and under pressures of 4 to 10 bar, the reaction mixture being pump-circulated via an inline disperser. The sirup is directly introduced into the reactor. After the addition, the reaction mixture is subjected to an after-reaction to ensure that the glucose is completely reacted off.
The above-mentioned processes have disadvantages which seriously limit their commercial value. If the glucose sirup is introduced into the mixture of alcohol and acidic catalyst too quickly, it is not adequately dispersed, even with intensive stirring. Instead, the glucose sirup passes through a tacky/viscous intermediate stage with release of water, resulting in the formation of agglomerates which are virtually impossible to redisperse. These agglomerates tend to settle mainly on the stirrer unit and cause it to stick and, in extreme cases, to clog up. In addition, another part of the non-dispersed, lumpy starting material usually adheres to the heated reactor wall so that partial carbonization can occur. The acetalization reaction between glucose and alcohol clearly cannot take place optimally where this procedure is adopted. For the most part, the reaction does not take place during the addition, but only during the after-reaction which, for this reason, has to be significantly prolonged.
The problem in question is normally avoided by adding the glucose sirup slowly, i.e. at such a rate that no lumps are formed, instead a single-phase homogeneous system is present or a more or less fine dispersion is formed in which the acetalization can take place without difficulty. Although the after-reaction time can be shortened to an economically acceptable level in this way, the addition time is significantly prolonged so that, in all, long reactor possession times are again the outcome.
Another disadvantage of the long reaction times is that the end reaction products have comparatively high contents of polyglucose and other unwanted secondary products, such as dialkyl ethers for example, and in addition can be seriously discolored by the severe exposure to heat.
Now, the problem addressed by the present invention was to provide an improved process for the production of lower alkyl oligoglucosides which would be free from the disadvantages mentioned above.